Merge pull request #2385 from henbos/191127_MandatoryGetSSRC

Note that getSynchronizationSources works without sinks

webrtc.html

@@ -7267,6 +7267,11 @@ <h2>Methods</h2>
       in order of ascending RTP timestamps. The user agent MUST keep information
       from RTP packets delivered to the <code><a>RTCRtpReceiver</a></code>'s
       <code><a>MediaStreamTrack</a></code> in the previous 10 seconds.</p>
+      <div class="note">Even if the <a>MediaStreamTrack</a> is not attached to
+      any sink for playout, <code>getSynchronizationSources</code> and
+      <code>getContributingSources</code> returns up-to-date information as long
+      as the track is not ended; sinks are not a prerequisite for decoding RTP
+      packets.</div>
       <div class="note">As stated in the <a href="#conformance">conformance
       section</a>, requirements phrased as algorithms may be implemented in
       any manner so long as the end result is equivalent. So, an